Stabilizing circuit for frequency-modulated oscillators



Dec. 20, 1949 H STABILIZING CIRCUIT FOR FREQUENCY MODULATED OSCILLATORS Flled Aprll 26, 1946 M w 5 M M Patented Dec. 20, 1949 STABlLIZIN-G CIRCUIT FOR FREQUENCY- MODULATED DSCILLATORS Gerard Helm, Eindhoven, Netherlands, minor, by mesne assignments, to Hartford National Bank and Trust Company, Hartford, Conn., as

trustee Application April 26, 1946, Serial No. 665,008 In the Netherlands March 24, 1942 Section 1, Public Law 690, August 8,1946 Patent expires March 24, 1962 8 Claims. 1

In order to stabilize the mean frequency in frequency modulation system, it is known to utilise a comparison source of constant frequency and to derive from the difference frequency a control voltage by which the frequency of the frequency modulation wave generator is controlled.

The present invention relates to a circuit of this kind and has for its purpose to provide a circuit of a high control sensitivity accompanied by insensitivity to amplitude variations and/or distortions which occur in the system.

The invention is based on the recognition of the fact that when a condenser which is charged in one sense by the source to be frequency-controlled in such manner that a substantially constant charge is brought on the condenser per cycle, and is charged in the other sense by a constant source of direct voltage via a resistance the condenser will assume either a positive or a negative voltage, depending on whether the frequency to be controlled is greater or smaller than the desired frequency. For the present, the amplitude of the source is assumed to be constant. If provisions are made whereby the value c of this source of direct voltage increases or decreases with the amplitude of the alternating voltage to be controlled, the sign of the charge of the condenser becomes independent of the last-mentioned amplitude. These effects may be obtained, for example, by causing the charge in the first-mentioned sense to be effected via a charging condenser and seeing to it that the product of the capacity of this charging condenser of the above-mentioned resistance and of the exact mean frequency=1.

In order that the invention may be more clearly understood and readily carried into effect, it will be described'more fully by reference to the accompanying drawing.

Figs. 1, 2 and 3 are diagrams to make the principle of the invention clear.

Fig. 4 shows a constructional example thereof.

In Fig. 1, E represents the value of the alternating voltage whose frequency is to be controlled. To this end, a control condenser C2 is charged in the one sense via a charging condenser C1 and a diode D2 and charged in the opposite sense by a battery B1 via a resistance R1.

A charging diode D1 for the charging condenser is connected in parallel with the source of supply E and the charging condenser C1.

The charge of the condenser C1 increases as the frequency of E is greater, whereas the charge in the other sense is to be considered as constant. When the charging condenser 01 is very small as compared with the control condenser 02, the charge per cycle on C2 is solely dependent on the amplitude of E.

The resulting charge of 02 will be nil, when )C'1R1=1, f representing the frequency. When I increases, C2 will. be charged in the one sense, and when I decreases, C2 will be charged in the opposite sense. The voltage e which appears across C2 may consequently be used as the control voltage.

Meanwhile in the voltage e will also depend on the amplitude value of E which can never be considered wholly constant even with the use of a limiter. A device for making e also independent of amplitude variations of E is shown in Fig. 2. This device differs from Fig. 1 in that the constant source of voltage B1 is now also made variable in accordance with the amplitude value E, since the battery B1 is replaced by a condenser G3 which is charged by the source of voltage to be controlled through a diode D3. Thus, the charge of the control condenser C2 in both senses is made de-, pendent on the amplitude of the voltage E, the variations in amplitude are nullified and have little or no influence on the sign of the control voltage e.

In the foregoing it has always been assumed that the voltage E is exactly sinusoidal, at least symmetrical relatively to the zero line. In reality this will not generally be the case. In order to make the control voltage independent of irregularities, the zero line may be displaced upwardly or downwardly until it becomes a tangent of the voltage curve. In Fig. 3 this is obtained by means of a condenser C4 and a diode D4.

The condenser C2, together with the two seriesconnected diodes D1 and D2. is located in a closed circuit and may consequently be charged in one direction only, since charges of opposite sign immediately leak away through the diodes. To correct this effect a counter E. M. F. may be included somewhere in the circuit. In this case, however, the voltage which occurs across the condenser G2 with the desired value of the frequency would not be equal to zero, as is most desirable for the control, unless an E. M. F. of equal value is switched-in on charging in the opposite sense. a

A device serving thereto is shown in Fig. 4.

In the anode conductor of the diode D1 and in the cathode conductor of the diode D4 there are included resistances R: and R3 of equal value,

" which are shunted by smoothing condensers.

case here considered the These resistances are traversed by a current originating from a source B2 of direct voltage and this in the opposite sense. Immaterial as to how great the value is of B2 and whether this value remains constant, the condenser C2 will not acquire any charge at the center frequency and with a deviation from this frequency will acquire a charge variable with the direction of this deviation in the one or in the other sense.

For obtaining a quick sensitive control the condenser C2 must not be given a too high value which involves however, on the one hand the danger that also the lower modulating frequencies must be suppressed, while on the other hand, the mean value of the voltage set up at C: becomes incorrect due to asymmetry. To obviate this drawback, these low modulating frequencies may be supplied once more in addition to the condenser C2 or be connected in series therewith. To this end a switch S1 may be opened and a doublepole switch S: be closed with the result that through a conductor m the lower modulating frequencies derived from the modulator part of the modulating oscillator MO via filter (not shown) are connected in series with the condenser C2.

This oscillator is frequency-modulated by a microphone MC, the central frequency being maintained constant with the aid of the control voltage set up across the condenser C2 and supplied to the oscillator through a conductor I. A crystal oscillator CO which in a mixing-stage ML with the oscillations of MO generates oscillations having the difference frequency serves as a comparison source. These oscillations are supplied via a limiter LM to the above-described portion of.

the circuit, where the control voltage for the oscillator is derived therefrom.

By way of example the following numerical values are given for the diiferent circuit elements of Fig. 4:

Mean frequency fm 69,000 cycles/sec. .Condenser Cl 0.005 pF Condenser C2 0.1 F Condenser C3 1 l Condenser C4 24 F Resistance R1 600,000 ohms Resistance R2 4,700 ohms Resistance Ra 4,700 ohms Voltage B2 volts I claim:

1. A circuit arrangement for stabilizing the mean frequency of a frequency-modulated wave at a predetermined center frequency, comprising a control capacitor, means to cyclically charge said control capacitor in one sense to a given value, means responsive to said frequency-modulated wave to cyclically charge said capacitor in the opposite sense to a second value proportional to the instantaneous mean frequency of said frequency-modulated wave during alternate half wave periods of said wave, and means responsive to the resulting charge difference on said capacitor to return the instantaneous mean frequento the predetermined center frequency of said frequency-modulated wave.

2. A circuit arrangement for stabilizing the mean frequency of a frequency-modulated wave at a predetermined center frequency, comprising a control capacitor, means to cyclically charge said control capacitor in one sense to a predetermined value, means responsive to said frequencymod-ulated wave to cyclically charge said capacitor in the opposite sense to a second value proportional to the instantaneous mean frequency us. said frequency-modulated wave during alternate half wave periods of said wave, said second value being equal to said predetermined value at said center frequency, and means responsive to the resulting charge difference on said capacitor to return the instantaneous mean frequency to the predetermined center frequency of said frequency-modulated wave.

3. A circuit arrangement for stabilizing the mean frequency of a frequency-modulated wave at a predetermined center frequency, comprising a control capacitor, means to derive a potential proportional to the amplitude of said frequency-modulated wave, means to apply said potential to cyclically charge said control capacitor in one sense to a first value, means responsive to said frequency-modulated wave to cyclically charge said capacitor in the opposite sense to a second value proportional to the amplitude and to the instantaneous mean frequency of said frequency-modulated wave during alternate half wave periods of said wave, and means responsive to the resulting charge difference on said capacitor to return the instantaneous mean frequency to the predetermined center frequency of said frequency-modulated wave.

4. A circuit arrangement for stabilizing the mean frequency of a frequency-modulated wave at a predetermined center frequency, comprising a control'capacitor, means to derive a first potential proportional to the amplitude of said frequency-modulated wave, means to apply said first (ill potential to cyclically charge said control capacitor in one sense to a first value, means comprising a charging capacitor and a rectifier connected in series to derive a second potential proportional to the amplitude and to the instantaneous mean frequency of said frequency-modulated wave, the capacity-of said charging capacitor being low with respect to the capacity of said control capacitor, means comprising a second rectifier poled opposite to said first rectifier to discharge said charging capacitor, means to apply said second otential to cyclically charge said con trol capacitor in the opposite sense to a second value during alternate half wave periods of said wave, and means responsive to the resulting charge difierence on said control capacitor to return the instantaneous mean frequency to the predetermined center frequency of said frequency-modulated wave.

5. A circuit arrangement for stabilizing the mean frequency of a frequency-modulated wave at a, predetermined center frequency, comprising a control capacitor, means to derive a first potential proportional to the amplitude of said frequency-modulated wave, means to apply said first potential to cyclically charge said control capacitor in one sense to a first-value, means comprising a charging capacitor and a rectifier connected in series to derive a second potential proportional to the amplitude and to the instantaneous mean frequency of said frequency-modulated wave, the capacity of said charging capacitor being low with respect to the capacity of said control capacitor, means comprising a second rectifier poled opposite to said first rectifier to discharge said charging capacitor, 9. source of voltage operative in opposition to said second rectifier, means to apply said second potential to cyclically charge said control capacitor in the opposite sense to a second value, and means responsive to the resulting charge difference on said control capacitor to return the instantaneous mean frequency to the predetermined center frequency of said frequency-modulated wave.

6. A circuit arrangement for stabilizing the mean frequency of a frequency-modulate wave at a predetermined center frequency, comprising a control capacitor, means comprising a first rectifier and a second capacitor connected in serice to derive a first potential proportional to the amplitude of said frequency-modulated wave, a second rectifier poled opposite to said first rectifler and shunted across said series combinetion of said first rectifier and said aecond'capacitor, a first source of direct voltage operative in opposition to said second rectifier, means in cluding a resistor to apply said first potential to cyclically charge said control capacitor in one sense to a first value, means comprising a charging capacitor and a third rectifier connected in series to derive a second potential proportional to the amplitude and to the instantaneous mean frequency of said lrequency-mcdulated wave, the capacity of said charging capacitor being low with respect to the capacity of said control capac itor, means comprising a fourth rectifier poled opposite to said third rectifier to discharge said charging capacitor. a second scarce of direct voltage operative in opposition to said fourth rectifier, means to apply said second potential to cyclically charse said control capacitor in the opposite sense to a second value. and means responsive to the resulting charge difference on said control capacitor to return the instantaneous mean frequency to the predetermined center irequency of said frequency-modulated wave.

7. A circuit arrangement for stabilising the mean frequency of a frequency-modulated wave at a predetermined center frequency, comprising a control capacitor, means comprising a first rectifier and a second capacitor connected in series to derive a first potential proportional to the amplitude of said frequency-modulated wave. a second rectifier poled opposite to said first rectifier and shunted across said series combination. means including a first resistor to apply said first potential to cyclically charge said control capacitorinonesensetoafirstvalue,meanscomprisingacharsinscapacitorandathirdrectifier connected in series to derive a second potential proportional to the amplitude and to the instantaneous mean frequency of said frequencymodulated wave, the capacity of said charging capacitor being low with respect to the capacity of said control capacitor, means comprising a fourth rectifier poled opposite to use third rectifier a discharge "said charging capacitor, a second resistor, athird resistor havingavslueequsltothatofsaidsecondresistor :andconnectedinscriesthmwitnasourceof direct voltage connected across said first and said second resistors, means to apply the voltage drop across said second resistor in opposition to said second rectifier, means to apply the voltage drop across said third resistor in opposition to said fourth rectifier, means to apply said second potential to cyclically charge said control capacitor in the opposite sense to a second value, and means responsive to the resulting charge ditference on said control capacitor to return the in stantaneous mean ireouency to the predator mined center frequency of said ireouency mccl uleted wave.

a. A circuit rearrangement for stabilising the mean frequency" or a. frequency-modulated wave at a predetermined center irequency, comprising source of modulating voltages covering hand ct i'requencles, means to select those modulation! voltages appearino in the lowermost end at the hand oi ireouencles, a control capacitor, means to first potential proportional to the anu plitude oi iconoclast-modulated wave, means to apply said. first potential to cyclically charge said control capacitor in one sense to a first value, means comprising a charging capacitor and a rectifier connected in series to derive a secand potential proportional to the amplitude and to the instantaneous mean frequency oi said frequency-modulated wave, the capacity oi said charging capacitor heinu low with respect to the capacity of said control capacitor, means comprising a second rectifier poled opposite to said first rectifier to discharge said charging capitoltor, means to apply said second potential to cyclically charge said control capacitor in the opposite sense to a second value, means to apply said selected modulating voltages in series with the charge on said control capacitor, and means responsive to the algebraic sum of the resulting charge difference on said control capacitor and said selected modulating voltages to return the instantaneous mean frequency to the predetermined center frequency of said frequency-modulated wave.

GERARD mp.-

REFERENCES crrsn The following references are of record in th file of this patent:

UNITED STATES PATENTS Number Name Date 1,958,994 Green May 15, 198i 2,131,443 Kummerer et al. Sept 27, 1938 2,214,573 Booth Sept. 10, 1940 2,279,660 Crosby Apr. 14, 1942 2,335,796 Bchraeder et al. Novi 80, 1943 

